Declining Importance of Trilobites in Ordovician Nearshore Paleocommunities: Dilution or Displacement?

Abstract

Declining importance of trilobites was a key feature of Ordovician community evolution. Previous work has shown that replacement of trilobitedominated paleocommunities by molluscand brachiopod-rich paleocommunities was diachronous, and began in nearshore environments. However, the processes responsible for these changes remain unclear. New data from northern North America indicate that trilobite species diversity in nearshore settings maintained a constant, low level between the Late Cambrian (Marjuman) and Middle Ordovician (Whiterockian). As the total number of species of other groups, especially molluscs, increased in nearshore environments, the relative importance of trilobites in these paleocommunities declined through a process of dilution, rather than actual displacement. The apparent offshore retreat of trilobite-dominated paleocommunities is also at least partly a reflection of this dilution process: through OFFSHOREI the Cambrian and Ordovician, trilobite assemblages tended to be most speciose in offshore environments, so that the apparent rate of dilution proceeded more slowly than in the nearshore. Thus, trilobite-dominated paleocommunities maintained their integrity longer in the offshore, and this produced an overall pattern of diachronous replacement. Similarly, progressive offshore replacements of other community types in younger strata may also be dilution phenomena related to species diversity gradients.